1. Field of the Invention
The invention relates generally to a catalyst deterioration detecting apparatus that uses a catalyst having an oxygen storage capability. More specifically, the invention relates to a catalyst deterioration detecting apparatus that detects deterioration of a catalyst that purifies exhaust gas of an internal combustion engine.
2. Description of Related Art
A catalyst used to purify exhaust gas is disposed in an exhaust passage of an internal combustion engine for a vehicle. This catalyst is capable of storing an appropriate amount of oxygen. When the exhaust gas that flows into this catalyst contains unburned components such as hydrocarbons HC and carbon monoxide CO and the like, the catalyst uses this stored oxygen to oxidize them. Also, when the exhaust gas contains oxides such as nitrogen oxide NOx and the like, the catalyst reduces them and stores the resultant oxygen.
The catalyst disposed in the exhaust passage of an internal combustion engine for a vehicle aims to purify the exhaust gas as described above. Therefore, the purification capability of the catalyst is largely affected by its oxygen storage capability. Accordingly, the deterioration state of the purification capability of the catalyst is determined by the maximum amount of oxygen able to be stored by the catalyst, i.e., by the oxygen storage capacity. As a result, in order to determine the deterioration state of the catalyst, it is necessary to accurately detect the oxygen storage capability of the catalyst.
As related art, an apparatus has been known that accurately detects the oxygen storage capability of the catalyst by oscillating the air-fuel ratio of the exhaust gas flowing into the catalyst (hereinafter referred to as the “exhaust air-fuel ratio”) back and forth between rich and lean so as to increase and decrease the amount of oxygen stored in the catalyst and detecting the exhaust air-fuel ratio of the exhaust gas flowing out on the downstream side of the catalyst with an air-fuel ratio sensor. The deterioration of the catalyst from the detected oxygen storage capability is then determined (Japanese Patent Application Laid-Open Publication Nos. 5-133264 and 5-209510 and the like). Japanese Patent Application Laid-Open Publication No. 5-133264, for example, discloses an apparatus that detects the oxygen storage capacity of a catalyst disposed in an exhaust passage by forcing exhaust gas, with the rich or lean air-fuel reaction, to the internal combustion engine. Exhaust gas having a shortage of oxygen that contains unburned components, such as HC and CO, is supplied to the catalyst while the air-fuel ratio is rich. When this kind of exhaust gas having a shortage of oxygen flows into the catalyst, the catalyst discharges oxygen stored therein in an attempt to purify the exhaust gas. Accordingly, when exhaust gas having a shortage of oxygen flows into the catalyst and oxygen continues to be discharged from the catalyst over an extended period of time, the catalyst eventually discharges all of its oxygen such that it is no longer able to oxidize the HC and CO. This state of the catalyst will hereinafter be referred to as “minimum stored oxygen state”.
Conversely, exhaust gas having an excess of oxygen that contains NOx flows into the catalyst while the air-fuel ratio is lean. When this kind of exhaust gas having an excess amount of oxygen flows into the catalyst, the catalyst stores the excess oxygen in the exhaust gas in an attempt to purify the exhaust gas. Accordingly, when exhaust gas having an excess amount of oxygen flows into the catalyst and oxygen continues to be stored in the catalyst over an extended period of time, the catalyst eventually becomes full of oxygen such that it can no longer reduces the incoming NOx and therefore can no longer purify the exhaust gas. This state of the catalyst will hereinafter be referred to as “maximum stored oxygen state”.
The apparatus according to the foregoing related art controls the air-fuel ratio of the mixture supplied to the internal combustion engine so as to repeatedly put the catalyst in the minimum stored oxygen state and the maximum stored oxygen state, alternating between the two states. The oxygen storage capacity of the catalyst is then obtained by integrating the amount of oxygen stored in the catalyst during the process in which the catalyst shifts from the minimum stored oxygen state to the maximum stored oxygen state, or by integrating the amount of oxygen discharged from the catalyst during the process in which the catalyst shifts from the maximum stored oxygen state to the minimum stored oxygen state. The foregoing apparatus determines whether the catalyst is normal or is deteriorating based on whether the oxygen storage capacity obtained in the foregoing manner is larger than a predetermined determination value.
In this apparatus, the air-fuel ratio of the mixture is switched from lean to rich after the catalyst reaches the maximum stored oxygen state and from rich to lean after the catalyst reaches the minimum stored oxygen state. For a certain period of time after the catalyst has switched from lean to rich, exhaust gas having an excess amount of oxygen continues to flow into the catalyst, which is in the maximum stored oxygen state. As a result, unpurified exhaust gas having an excess amount of oxygen flows out downstream of the catalyst during this period. Similarly, for a certain period of time after the catalyst has switched from rich to lean, exhaust gas having a shortage of oxygen flows out downstream of the catalyst, which is in the minimum stored oxygen state.
One conceivable method to prevent unpurified exhaust gas from being discharged into the atmosphere (i.e., making emissions worse) is, for example, to dispose a downstream side catalyst downstream of that catalyst. This configuration effectively prevents exhaust emissions from becoming worse by treating the unpurified exhaust gas that flows out from the catalyst on the upstream side with the downstream side catalyst.
Even when the downstream side catalyst is provided, however, if exhaust gas having an excess amount of oxygen flows out from the catalyst on the upstream side when the downstream side catalyst has stored substantially all of the oxygen it can store, that exhaust gas would pass straight through the downstream side catalyst and be discharged into the atmosphere as it is. Similarly, when the downstream side catalyst has discharged substantially all of its oxygen, if exhaust gas having a shortage of oxygen flows out from the catalyst on the upstream side, that exhaust gas would be discharged as it is into the atmosphere without being purified even by the downstream side catalyst.
In this way, when attempting to determine the deterioration of a catalyst by forcefully oscillating the air-fuel ratio back and forth between rich and lean, there is still a possibility of the air-fuel ratio being disturbed in the deterioration determination process, thereby temporarily worsening the exhaust emissions, when only providing a downstream side catalyst further downstream of the catalyst of which determination is being performed.